This invention relates to the field of polyhydroxyalkanoate polymers, articles made therefrom and melt processing methods. More specifically, the invention pertains to melt processing and cast film extrusion of polyhydroxyalkanoates which contain a novel combination of nucleant and plasticizer for enhancing chain mobility, crystallization rates in order to improve processibility in conventional melt processes.
Polyhydroxyalkanoates (PHAs) and other thermoplastic polyesters represent potential raw materials for a myriad of useful products. Examples include melt-spun fibers from which non-woven products can be produced for medical gowns and masks, blown and cast films for compostable grocery and garbage bags, injection-molded bottles for health and personal care products and extrusion coatings on paper/paperboard for biodegradable/compostable fast-food containers. In the processes to produce PHA products, it is crucial to achieve line speeds, cycle times, and other processing parameters that are economically desirable.
Polyhydroxyalkanoate copolymers are extremely sticky when melt processed due, at least in part, to extremely slow crystallization rates of their crystalline phase domains. This sticky or xe2x80x9ctackyxe2x80x9d behavior leads to an inability to process the polymer through any melt processing equipment, including extrusion, compounding, film and fiber operations. Unmodified polymer has a strong tendency to stick to all pieces of machinery, regardless of the material of construction. The polymer also has a strong tendency to stick to itself and to human skin when touched. The stickiness or tackiness gradually disappears in a matter of minutes to hours. However, this time frame is significantly long for any conventional processing techniques, which generally require the polymer to become non-tacky within a matter of a few seconds.
Previous work has shown that addition of a crystallization nucleant to some compositions of polyhydroxyalkanoate copolymers can increase crystallization rates to a point where melt processibility is acceptable. In addition, such nucleants can sometimes improve the physical and mechanical properties of the processed articles. Conventional nucleating agents include, for example, talc, micronized mica, calcium carbonate, boron nitride (see, for example, EP 0291024) and ammonium chloride (see, for example, WO 9119759).
U.S. Pat. No. 5,296,521 describes polyester compositions having increased crystallization rates comprising thermoplastic polyester resins and 0.5 to about 5 weight percent of nucleating agent of the formula RO[P(O)(Ph)(CH2)mO]nH where R is an alkali or alkaline earth metal; m is 1, 2, or 3; n takes an average value within the range of 1 to 5. The nucleating agent can be optionally mixed with the acid or ester form providing at least 50 mole percent of the nucleating agent is in the salt form. The nucleating agent is preferably in the form of the sodium salt (e.g., sodium salt of hydroxymethylphenyl phosphonic acid or sodium salt of oligomethylene phenyl phosphinic acid).
U.S. Pat. No. 4,536,531 describes use of carboxylic salts of metals of Group I and II in the Periodic Table as nucleating agents for polyesters exemplified by metal salts of aliphatic monocarboxylic acids such as acetic acid, propionic acid, caproic acid, palmitic acid, stearic acid, oleic acid, behenic acid and montanic acid. Suitable metals are sodium, potassium, lithium, magnesium, calcium, barium and zinc. In these carboxylic acid salts, it is unnecessary that all the carboxylic groups be converted into salt form, but a part of the carboxyl group may be in a salt form and the remaining groups may be in a free acid or ester form.
Several references disclose the use of organophosphorous compounds as nucleants. U.S. Pat. No. 5,061,743 discloses a preferred polyhydroxyalkanoate nucleant made by dry blending cyclohexylphosphonic acid and zinc stearate with polyhydroxybutyrate-co-valerate. The nucleant is disclosed as particularly advantageous for the nucleation of polyhydroxybutyrate-co-valerate having high hydroxyvalerate content. WO 9905208 discloses that organophosphorous compounds having at least two phosphonic acid moieties can be used as nucleants for polyhydroxyalkanoates and other thermoplastic polyesters.
Although many of these compounds have shown effectiveness in increasing the nucleation density of polyhydroxyalkanoate, and therefore crystallization rates, certain disadvantages have been associated with their use. Dispersion of particulate nucleants, for example, has been problematic because agglomeration frequently occurs during processing which can generate regions of stress concentration and inhomogeneity in molding. In addition, nucleants such as boron nitride have been found to act as pigments in some situations, particularly in films and injection moldings, giving rise to opaque products where transparent products are generally desired. Further, some nucleant systems include constituents which may be environmentally and toxicologically undesirable.
Furthermore, the polymers of the previous references tend to be copolymers that do not contain a comonomer that effectively increases the amorphous character or decreases crystallization rates of the polymer. The resulting polymers are often brittle and lead to undesirable properties. Polyhydroxyalkanoate copolymers that contain more modifying comonomers, which lead to a significant amount of amorphous phase, tend to be more desirable polymers because they exhibit a high level of toughness and elastic resilience. However, the large amount of amorphous phase contained in these polymers is not conducive to good crystal formation or rapid crystallization rates. Furthermore, addition of nucleants to these polymers generally does not increase the amount of crystallization or the crystallization rate enough to make melt processing of these polymers feasible. Thus, there is a need for benign and cost-effective nucleant systems which allow for the production of polyhydroxyalkanoate resins having moderate to high crystallinity, excellent moldability, mechanical strength and dimensional stability.
The problem to be solved, therefore, is to provide a polymeric composition that produces tough, flexible polyhydroxyalkanoates that can be readily and quickly processed into film-based products. Another objective of this invention is to provide a method for continuous melt extrusion of these polyhydroxyalkanoates. Yet another object of the present invention is to provide a method for continuous cast film production using these polyhydroxyalkanoates.
The invention provides a polyhydroxyalkanoate copolymer composition which can be processed into film-based products, extruded and molded articles, and coatings, comprising: (a) a polyhydroxyalkanoate copolymer; (b) a nucleant; and (c) a plasticizer, and a method of making same.
In a preferred embodiment, unique combinations of either poly-3-hydroxy(butyrate-co-octanoate) or poly-3-hydroxy(butyrate-co-hexanoate) are polymerized with polyhydroxybutyrate (nucleant) and either methyl laurate or dibutylmaleate (plasticizer).